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Advances in Space Research 38 (2006) 2068–2074

Synchrotron X-rays in situ analysis of extraterrestrial grainstrapped in aerogel

J. Borg a,*, Z. Djouadi a, F. Grossemy a, D. Eichert b, G. Martinez-Criado b,A.J. Westphal c, D. Deboffle a, C.J. Snead c, A. Somogyi b

a Institut d�Astrophysique Spatiale, Batiment 121, Campus, 91405 Orsay, Franceb European Synchrotron Radiation Facility, Grenoble, France

c Space Sciences Laboratory, Berkeley, USA

Received 4 October 2004; received in revised form 20 December 2004; accepted 18 April 2005

Abstract

In January 2006, the NASA Stardust mission will return, trapped in aerogel, the first samples of cometary and contempo-

rary interstellar dust. Their analyses will require high spatial resolution techniques. An example is Synchrotron X-ray analysis,

which could be useful for in situ identification of the captured grains. In the case of aerogel, it is possible to optically scan the

samples and locate the trapped grains, which can then be extracted in pieces of aerogel, called ‘‘keystones’’, a few hundreds of

microns large and self-supported on microforklifts that contain one incident grain and its penetration track. The non-destruc-

tive identification of the material distributed inside a keystone can be completed using Synchrotron X-ray microbeams either in

the fluorescence or the absorption mode. In order to develop an analytical protocol, we analysed a keystone containing a dust

grain originating from the chondritic swarm identified in the Orbital Debris Collection Experiment (ODCE) that was deployed

for 18 months outside the Russian space station MIR. The results we present concern l-fluorescence mappings and XANES at

the Fe K-edge measurements. They indicate a very strong tendency of the incident particle to break up into many fragments

during the slowing in the aerogel. Furthermore, a structural evolution of the grain is also possible, during the slowing down in

the aerogel. These non-destructive techniques might be among the only ones to be used for analysing the interstellar grains

trapped in the Stardust aerogel and for those cometary grains that might have lost most of their material along the penetration

track.

� 2005 COSPAR. Published by Elsevier Ltd. All rights reserved.

Keywords: Synchrotron X-rays analyses; Stardust analytical procedures; Cometary grains in aerogel

1. Introduction

In January 2006, the NASA Stardust mission willreturn from its journey to comet Wild2, with the first

0273-1177/$30 � 2005 COSPAR. Published by Elsevier Ltd. All rights reser

doi:10.1016/j.asr.2005.04.058

* Corresponding author. Tel.: + 33 1 6985 8632.

E-mail addresses: janet.borg@ias.u-psud.fr (J. Borg), zahia.djouadi

@ias.u-psud.fr (Z. Djouadi), faustine.grossemy@ias.u-psud.fr (F.

Grossemy), diane.eichert@esrf.fr (D. Eichert), gema.martinez-criado

@esrf.fr (G. Martinez-Criado), westphal@ssl.berkeley.edu (A.J. West-

phal), dominique.deboffle@ias.u-psud.fr (D. Deboffle), chalcedony

@hotmail.com (C.J. Snead), somogyia@esrf.fr (A. Somogyi).

samples of cometary and contemporary interstellar

dust trapped in aerogel collectors. It is believed that

comet Wild2 is composed of extremely primitive mate-rial, practically not altered since its formation and its

accretion in a solid body, so its analysis should give

important information on the early solar nebula.

From dust measurements in the comet�s coma by the

Dust Flux Monitor Instrument, it is possible to esti-

mate the number of particles having impacted the

aerogel collectors to be around 3000 particles of diam-

eter 15 lm or larger (Tuzzolino et al., 2004). The

ved.

J. Borg et al. / Advances in Space Research 38 (2006) 2068–2074 2069

importance of the information contained in the grains

is unequalled and for the first time we will have access

to samples with a guaranteed cometary or interstellar

origin for analysis in the laboratory (Brownlee et al.,

2004).

The analyses performed on these micron-sized sam-ples will require high resolution analysis techniques,

that do not modify the structure and the composition

of the grain, still embedded in the aerogel; we then

speak of non-destructive in situ analyses of the trapped

grain, for which the signal should not be disturbed by

the surrounding material. Among the techniques that

will eventually be used, those based on the use of Syn-

chrotron X-ray radiation are advantageous for the fol-lowing reasons: (i) the lateral resolution is of 1 lm or

less, and mapping of grains a few microns large is pos-

sible; (ii) at incident energies of a few keV, aerogel is

transparent to the beam which can be focused on the

absorbing grain; (iii) with the 3rd generation machines,

high fluxes of microbeams can be obtained, consider-

ably lowering the necessary integrated time for reason-

able counting. Furthermore, with the same microbeam,different analytical techniques can be performed, lead-

ing to complementary information on the grain. For

instance, in a way that is described in this paper, spe-

cific elemental mapping obtained by fluorescence of

the atoms constituting the target gives information on

its composition and the repartition of the elements in-

side the sample, while absorption techniques like

XANES can lead to information on the oxidationstates of the various atoms. At the European Synchro-

tron Radiation Facility (ESRF), these techniques are

available on the complementary beamlines ID21 and

ID22 (ESRF, 2004).

In order to test the feasibility of these techniques

and to develop them for the return of the Stardust

collectors, we used a keystone extracted from the

Orbital Debris Collection Experiment (ODCE) fromNASA (Horz et al., 2000). We describe here the ana-

lytical procedures and the first results obtained. It

must be noted that these results are still very prelimin-

ary, because we did not optimize the counting rates

but chose to pursue a general investigation of the

sample by cartography obtained by fluorescence and

by spectroscopic techniques. Only one keystone was

investigated, and at evidence more must be analysed.We can already be sure that these techniques are

among the most fruitful and might even be the only

ones possible in case of interstellar micron and submi-

cron grains as well as for cometary grains if they show

up as porous aggregates that will lose most of their

material during their slowing in the aerogel. After a

brief description of the experimental techniques

used for the keystone�s analysis, we finally give themain results obtained and the conclusions that can

be drawn.

2. Preparation of the samples

2.1. The Stardust and ODCE collections

The Stardust mission launched in 1999 successfully

collected dust from comet Wild2 in January 2004 andis on its way back with its harvest of interstellar and

cometary grains collected in 1039 cm2 of aerogel (Tsou

et al., 2003). The particles to be collected hit the surface

at hypervelocities and it is known from previous exper-

iments and collections in Low Earth Orbits (LEOs) that

they produce carrot shaped tracks; roughly speaking,

the length of the track is more than 10 times bigger than

the size of the particle, as suggested by calibration(Burchell et al., 2001), although very recent modeling

of the interaction of hypervelocity particles with aerogel

lack in giving a direct relation between the size and/or

the velocity of the particle and the size (length, diameter)

of the track (Dominguez et al., 2004).

In order to predict what will be found in the Stardust

aerogel tiles, and to prepare the analytical procedures to

be used, we must mainly rely on recent collection exper-iments in LEO, in which aerogel was used as the collect-

ing material. Such an experiment that is now currently

investigated is the NASA ODCE, that was exposed out-

side the MIR station for 18 months, in 1996–1997 for

the study of the orbital debris environment of the sta-

tion. Fortunately, the station was hit by a relatively

large chondritic particle and the resulting secondary par-

ticles were collected by some of ODCE aerogel tiles andare available for analysis (Horz et al., 2000).

2.2. The LCO1B2 keystone

Once the impact positions are identified by an opti-

cal investigation of the collectors, the procedure for

analysing the trapped grains first consists in extracting

the piece of aerogel containing one grain and its pene-tration track, called a ‘‘keystone’’, in order to analyse it

in situ by non-destructive techniques. It can eventually

be followed by an extraction of the bare grain for fur-

ther analyses. It is a difficult challenge to extract such a

piece of aerogel without destroying either the material

in the keystone, or the aerogel in the vicinity, that

might contain more precious grains to be analysed.

The extraction protocol is thoroughly described inWestphal et al. (2000). We have been working on one

such keystone, namely LC01B2 from ODCE, for which

an optical view is given in Fig. 1. The terminal grain

sitting at �250 lm from the surface of the aerogel is

<5 lm in size and a substantial amount of the initial

grain seems to be concentrated in the carrot shaped

track �150 lm long and �20 lm large. So, any analyt-

ical technique will have to consider analysing numerousmicron-sized grains dispersed in a small volume of

aerogel.

Fig. 1. Optical view of keystone LCO1B2, from ODCE, self supported on the microforklift. The terminal grain is <5 lm in size and a substantial

amount of the initial grain is seen in the first part of the penetration track.

2070 J. Borg et al. / Advances in Space Research 38 (2006) 2068–2074

3. Experimental analytical techniques

In order to characterize the grains in situ in the key-

stone, spectroscopic non destructive methods have to be

developed. Because of the small size of each individual

grain (less than 5 lm), Raman microspectroscopy is

not suitable, as the small signal eventually producedby the grain does not overlap the strong fluorescence sig-

nal delivered by the aerogel environment. However, this

technique has proved to be possible when larger grains

are concerned (Burchell et al., 2001, 2004). Also, IR mic-

rospectroscopy is not possible in the range 4000–

650 cm�1 actually available on the IR Nicolet micro-

scope that we use, because of the strong response of

aerogel to IR excitation at �1000 cm�1. Measurementsat lower wavenumbers (500 cm �1) have been performed

on grains trapped in aerogel by Keller and Flynn (2003)

and the mineralogical state of the grains could then be

characterized.

We favour the excitation of the keystone with X-ray

microbeams, such as those delivered by synchrotron

facilities, for the following reasons: (i) the aerogel is

transparent to the incident beam at the energies usedfor the characterisation of the grains (620 keV), so that

absorption only takes place where a trapped grain is

found; (ii) the incident beam can be focused down to

sizes of 1 lm on the sample and each individual grain

contained in the keystone can be identified and mapped

in order to scan eventual inhomogeneities; (iii) even with

the high flux beams used, the method is fully non

destructive, in the sense that it does not heat the sampleor irradiate it. First positive results in the use of these

techniques for locating and characterizing grains

trapped in low density materials were obtained by Flynn

et al. (1996) for Allende meteorite grains trapped inside

aerogel. Low density materials have been currently used

in the past years for collecting grains outside the MIR

station, and Synchrotron X-ray techniques have been

successfully used for the identification of the trapped

grains (Flynn et al., 2000, 2001; Borg et al., 2002).

3.1. Characteristics of beamlines ID21 and ID22 (ESRF)

We have tested the Synchrotron radiation techniquesat the European Synchrotron Radiation Facility (ESRF,

Grenoble, France), on the beamlines ID21 and ID22,

both devoted to high resolution microprobe techniques,

at, respectively, low incident energies (EID21 6 7.5 keV)

and high incident energies (EID22 P 7 keV) favouring

the characterisation of light elements (Z < 27) at ID21

and of heavy elements (20 < Z < 82) at ID22. The differ-

ent signals resulting from the interaction of X-rays withmatter can be detected with high spatial resolution on

the micrometer scale. The results presented in this paper

concern l-fluorescence analysis at energies of 10 keV on

ID22, 7.2 keV on ID21 and XANES (X-ray Absorbtion

Near Edge Structure) at the Fe Ka edge (7125 eV) anal-

ysis on ID21.

3.2. Analytical conditions on ID22

For analysis on ID22, the tips supporting the key-

stone sample can be fixed onto a goniometer head in or-

der to align them carefully in the high flux X-ray

microbeam. A Kirkpatrick–Baez (KB) mirror-pair is

used as focusing optics. This demagnifies the incident

beam to micron-sizes (H · V: 1 · 1 lm2) with a flux of

�1011 photons s�1 in the energy range used (between 7and 14 keV). A Si(Li) solid-state detector equipped with

an external collimator collects the X ray-fluorescence at

90� from the incident beam, at a sample-detector dis-

tance of �30 mm in order to reduce the scattering and

higher order fluorescence signals in the measured spec-

tra. Incident energies between 7 and 14 keV, as in our

J. Borg et al. / Advances in Space Research 38 (2006) 2068–2074 2071

experiment, are well adapted to the excitation of the K-

lines of all the elements down to Z = 18 (Ar) and of the

L-lines of the heaviest elements. In order to reduce the

air path between the sample and the detector and to im-

prove the efficiency for low Z elements, we used a He

chamber, which makes the identification of all elementsdown to Z = 14 (Si) possible. No information on the Si

content of the grain is of course possible, due to the high

Si background of the aerogel. We first proceeded to a

scan of the complete keystone at a lateral resolution of

5 lm/pixel, before focusing on the analysis of the ele-

ments distributed along the track and at its end, in the

terminal grain, at a lateral resolution of 1 lm/pixel.

For each scan, maps of the distribution of various char-acteristic elements (Fe, Ni, Cr, Mn, Ca, Au, and Zn) are

drawn, and a X-ray spectrum can be produced for each

pixel, so that compositions in main elements are known

for each absorbing region. Such mappings can be time-

consuming, if a high resolution is requested: more than

5 h are needed to scan the track region at 1 lm and 1 s/

pixel. So we decided, in this first investigation, not to

spend more than 0.5 s/pixel which of course does not al-low the possibility of analysing the trace elements.

3.3. Analytical conditions on ID21

This study was carried out at the X-ray Microscopy

Beamline (ID21) of the ESRF. For analysis on ID21,

the sample has to be placed under vacuum to see any

eventual light element (Z < Ca) present inside oursample. For that, it is necessary to extract the key-

stone from the microforklift and carefully place it

on a dedicated sample holder, between two thin ultra-

lene films. The Scanning X-ray Microscope (SXM) at

ID21 uses Fresnel zone plates as focusing optics to

generate a submicron X-ray probe, that is used to

investigate the sample with various contrast mecha-

nisms (including fluorescence, transmission and phasecontrast). The sample was mounted vertically on a

high precision piezo-electric stage and scanned hori-

zontally and vertically in a raster form under the

beam to produce a two-dimensional image. A high

purity Ge energy-dispersive detector (Princeton, Gam-

ma-Tech) mounted in the horizontal plane perpendic-

ular to the beam was used to collect the fluorescence

photons emitted. This geometry minimizes the contri-bution of elastic scattering. Provided the sample is

thin enough, a Si-photodiode mounted ‘‘downstream’’

of the sample can additionally detect the transmitted

signal. The energy range between 7 and 7.5 keV was

used to cover completely the K-edge of iron at

7112 eV. This energy region was obtained by means

of a fixed-exit double crystal Si(11 1) monochromator,

located ‘‘upstream’’ of the microscope, and which pro-vided the energy resolution of DE/E = 10�4 necessary

to resolve XANES figures. The SXM was operated

under vacuum to avoid the strong absorption of the

different fluorescence lines due to the air (Ar).

The combination of the high spectral resolution of

the monochromator and of the high spatial resolution

of the X-ray microprobe offers a unique tool for the

chemical mapping of elements using XANES spectralfeatures. Although the primary beam energy was set

around that of the Fe K-edge energy region, elements

with absorption edges at lower energies were also sub-

ject to excitation and emission of fluorescence pho-

tons. Thus, multichannel data collection of the

fluorescence photons emitted by the sample enabled

the element mapping of the different elements present

in the sample simultaneously with the iron maps. Themolecular environment of a given element can then be

characterized by tuning the energy of the probing X-

ray photons close to the absorption edge of the ele-

ment, here at 7.2 keV, just after the absorption edge

of iron. So, in a first step, chemical mappings were

realized to see the trace of iron inside the aerogel.

For selected areas of the aerogel, where fragments of

the incident grain are looked for, X-ray transmissionimages were produced at the submicron scale (beam-

size of 0.5 · 0.5 lm2) to reveal the distribution and

the micro-spatial structure of the penetration track.

The iron content of the sample was assessed by taking

X-ray fluorescence images on the iron resonance line.

Then spatially resolved XANES spectra for iron coor-

dination were collected at three locations in the key-

stone, identified after its l-fluorescence mapping: inthe terminal grain, and in small grains at the entrance

of the track and at the bottom of the track, showing

an enriched signal in Fe. Because of the low statistics

encountered, the data were only collected in the fluo-

rescence mode with the sample positioned 45� with re-

spect to the beam, as for the l-fluorescence

experiments. The beam size at the sample position

was 0.5 · 0.5 lm2 and the energy resolution was0.3 eV at the Fe K-edge. XANES spectroscopy pro-

vides direct structural information for the first and

more distant neighboring atoms around Fe. More pre-

cisely, the pre-edge structure which is located �15–

20 eV before the main K-edge crest (white line) of

Fe (7125 eV) is sensitive to Fe redox states and it

shifts towards higher energy with increasing oxidation

state (Wilke et al., 2001). For each XANES at the FeK-edge spectrum, we extracted, following the analyti-

cal method described by Wilke et al. (2001), the pre-

edge structure, or more precisely the position of its

centroid, signature of the Fe redox state. For energy

calibration, standard spectra were provided by siderite

(FeCO3) for Fe2+ (pre-peak position at 7112.16 eV)

and hematite (Fe2O3) for Fe3+ (pre-peak position at

7113.5 eV) in the same irradiation conditions, theseminerals being dispersed homogeneously as powder

on an ultralene film.

Fig. 3. X-ray fluorescence spectrum of the terminal grain. Peak

intensities have been normalized to [Fe Ka] = 100.

2072 J. Borg et al. / Advances in Space Research 38 (2006) 2068–2074

4. Main results

4.1. l-fluorescence results on ID22

We have performed mappings of the keystone

LCO1B2 in order to define the positions of the absorb-ing regions in the aerogel sample, the incident energy

being fixed at 10 keV. Fig. 2 shows the distribution of

the Fe Ka characteristic X-ray line for the complete key-

stone, at a lateral resolution of 5 lm. This figure must be

related to Fig. 1 which gives the optical view of the key-

stone. The microfork tips, which also show a consider-

able enrichment in Au and Zn, are clearly

distinguished, and the grain sitting at the end of thetrack is less than 5 lm large, while the track is about

200 lm long and 20 lm large ; clearly, along the first

‘‘carrot part’’ of the track, material has been deposited,

a terminal grain is visible and the slowing down between

the end of the carrot track and this terminal grain results

on a very thin track with practically no material depos-

ited. Gaining precise quantitative information about the

repartition of the different elements in the grain andalong the track is difficult due to the unknown self-

absorption within the grain. Thus, in this work, only

qualitative elemental identification was deduced from

the line-area of the fluorescence spectra, such as shown

in Fig. 3 for the terminal grain. Peak intensities have

been normalized to [Fe Ka] = 100. Each spectrum is ob-

tained by summing the contribution of all pixels consti-

tuting the grain. The main elements that can beidentified are Si, S, Ca, Cr, Mn, Fe, and Ni. The Cl sig-

nature that appears in the spectrum is present in all the

aerogel and is considered as an impurity; of course, no

information can be deduced from the Si abundance.

4.2. l-fluorescence results on ID21

As explained in Section 3.3, the l-fluorescence workon this beamline was mainly devoted to the identifica-

Fig. 2. Distribution of the Fe Ka characteristic X-ray line for the the compl

fluorescence intensity, blue colour low intensity. The analogy with the optic

tion of the positions of the different absorbing grains

in the aerogel, in order to perform the XANES spectros-

copy at the iron K-edge. We proceeded to Fe-mappings

with an incident energy of 7.2 keV and with a lateral res-

olution of 0.5 lm/pixel. A clearer identification of the

structure of the track and its constituting grains is then

possible. No X-ray spectrum was registered as the main

purpose of the work on this beamline was XANES studyon selected Fe hot spots, as described in the next

paragraph.

4.3. Fe-XANES results on ID21

Fig. 4 shows the normalized XANES spectrum col-

lected at the Fe Ka-edge for the terminal grain in

LCO1B2. The pre-edge feature can be observed and itscentroid position is found at 7112.3 eV. Pre-edges are

observed at all three locations where XANES spectra

were collected. Clearly, the centroid position varies from

a value of 7113.4 eV at the entrance of the track to val-

ues of 7111.8 eV at the end of the track and 7112.3 eV in

the terminal grain (Fig. 5). These values of the centroid

ete keystone, at a lateral resolution of 5 lm. Red colour indicates high

al view of the same keystone (Fig. 1) is unambiguous.

Fig. 4. XANES spectrum collected at the Fe Ka-edge for the terminal

grain. The spectrum has been normalized by taking a mean absorption

coefficient between 7200 and 7300 eV, so that the absorption is 1 in the

EXAFS region (see Wilke et al., 2001).

Fig. 5. Centroid positions for three Fe-rich grains: at the entrance of

the track (a), at the end of the track (b) and in the terminal grain (c).

J. Borg et al. / Advances in Space Research 38 (2006) 2068–2074 2073

positions of the pre-edges, obtained after calibration

with the two standards we used, namely siderite for

Fe2+ (7112.16 eV) and hematite for Fe3+ (7113.5 eV),indicate an evolution in the redox state of the Fe, along

the track from a Fe3+ value at the entrance of the track

to a Fe2+ value inside the aerogel.

5. Discussion and conclusions

In most studies regarding the identification of extra-terrestrial material, the chosen criterion is its chondritic

composition. Here, in a first step, we can only rely on

the [Fe]/[Ni] ratio for determining the origin of the grain,

roughly estimated by comparing the net peak intensity

of the Fe Ka line to that of the Ni Ka line on the differ-

ent spectra. This ratio, known to peak around 18 in

extraterrestrial matter and to be higher than 600 in ter-

restrial basalts or crust (Rietmeijer, 2004), is found to be

�17 in the LCO1B2 terminal grain, and in other grains

along the track. Such a value clearly suggests an extra-

terrestrial origin for the grain. This argument is corrob-

orated by the presence of other chondritic elements (S,

Ca, Cr, and Mn). A question to be addressed concernsthe abundance of the more volatile, lighter, elements,

like Mg, Al or Ca, that could not be identified in this

first set of experiments, neither on ID22 not adapted

to the identification of light elements nor on ID21 where

the counting rates were very small. A reasonable

hypothesis is that the incident grain is a coarse grain,

which suggests a similar composition at the entrance

of the aerogel for all the fragments. So a better knowl-edge of the variation in composition for the different ele-

ments would most probably help in improving the

understanding in the physics of the slowing down of a

hypervelocity grain in aerogel. This question is still open

despite the efforts done both in calibration and model-

ing. For the moment, it is possible to deduce neither

the size nor the velocity of the incident grain from the

shape of the track or the fragmentation of the incidentgrain.

Other information on the evolution of the incident

grain during its slowing down in the aerogel could be de-

duced from the Fe Ka-XANES results, as we have to ex-

plain an oxidation of Fe during the slowing down of the

grain and its fragmentation. We expect heating to rather

have an opposite effect. Two possible hypotheses are

suggested here: a combination of different minerals inthe primary grain, with different redox states or an inter-

action with the aerogel and more precisely the O present

in its structure. The answer is not possible with this sin-

gle experiment and more needs to be investigated, either

with this same sample, and much longer counting rates

or/and with new keystone samples on which the same

experiments are performed.

The results we have obtained are preliminary and wehave tentatively interpreted them in terms of under-

standing how to better handle the Stardust samples.

They already indicate the essential role that will be

played by Synchrotron X-ray techniques in the identifi-

cation of the composition of the incident grain. These

non-destructive techniques might be among the only

ones to be used for analysing the interstellar grains

trapped in the Stardust aerogel and for those cometarygrains that might have lost most of their material along

the penetration track. It seems reasonable to think that

the material that will be brought back by Stardust from

comet Wild2 will qualitatively resemble the material

analysed here: instead of dealing with a primary coarse

grain, it will most probably consist in an aggregate that

might disperse even more than what is observed here. So

this first step of conserving the total of the incident par-ticle in an individual keystone is crucial if one does not

want to introduce a bias in the analysis of the primary

2074 J. Borg et al. / Advances in Space Research 38 (2006) 2068–2074

grain. Until the return of the Stardust aerogels, big ef-

forts will have to be made in the extraction of the grains

and their complete characterisation.

Acknowledgements

We thank Mike Zolensky and Fred Horz for provid-

ing the ODCE aerogel tiles that were crucial for this pro-

ject. We thank Michelle Bonnin-Mosbah and Alexandre

Simionovici for fruitful discussions concerning the ana-

lytical treatment and the interpretation of XANES

spectra.

References

Borg, J., Quirico, E., Simionovici, A., Raynal, P.-I., Chevallier, P.,

Langevin, Y. Synchrotron radiation as a tool for in situ investi-

gation of extraterrestrial grains in low-density collectors: applica-

tons to the analyses of the PIE polymid foam targets. Plan. Sp. Sc.

50, 1055–1065, 2002.

Brownlee, D.E., Horz, F., Newburn, R.L., Zolensky, M., Duxbury,

T.C., Sandford, S., Sekanina, Z., Tsou, P., Hanner, M.S., Clark,

B.C., Green, S.F., Kissel, J. Surface of young Jupiter family comet

81P/Wild2: view from the Stardust spacecraft. Science 304, 1764–

1769, 2004.

Burchell, M.J., Creighton, J.A., Cole, M.J., Mann, J., Kearsley, A.T.

Capture of particles in hypervelocity impacts in aerogel. Meteorit.

Planet. Sci. 36, 209–221, 2001.

Burchell, M.J., Creighton, J.A., Kearsley, A.T. identification of

organic particles via Raman techniques after capture in hyperve-

locity impacts on aerogel. J. Raman Spectr. 35, 249–253, 2004.

Dominguez, G., Westphal, A.J., Jones, S.M., Phillips, M.L.F. Energy

loss and impact cratering in aerogels: theory and experiment.

Icarus 172 (2), 613–624, 2004.

ESRF, 2004 http://www.esrf.fr/exp_facilities/ID21/ and http://

www.esrf.fr/UsersAndScience/Experiments/Imaging/ID22/.

Flynn, G.J., Horz, F., Bajt, S., Sutton, S.R. In situ chemical analysis of

extraterrestrial material captured in aerogel. LPSC XXVII, 369–

370, 1996.

Flynn, G.J., Sutton, S.R., Horz, F. Synchrotron X-ray in situ analysis

of extraterrestrial particles collected in aerogel on the MIR space

station. LPSC XXXI, 2000, abstract #1457, LPI, Houston, CD-

ROM.

Flynn, G.J., Sutton, S.R., Horz, F. Synchrotron X-ray micro-

probe in situ trace element analysis of ‘‘swarm’’ particles

collected in aerogel on the MIR station: evidence for a CI-

like composition. LPSC XXXII, 2001, abstract #1398, LPI,

Houston, CD-ROM.

Horz, F., Zolensky, M.E., Bernhard, R.P., See, T.H., Warren, J.L.

Impact features and projectile residues in aerogel exposed on Mir.

Icarus 147, 559–579, 2000.

Keller, L.P., Flynn, G.J. Interplanetary dust and particles captured in

aerogel: far-infrared spectroscopy. Meteorit. Planet. Sci. 38, 2003,

abstract #5288.

Rietmeijer, F.J.M. Interrelationships among meteoritic metals, mete-

ors, interplanetary dust, micrometeorites and meteorites. Meteorit.

Planet. Sci. 39, 1375–1386, 2004.

Tsou, P., Brownlee, D.E., Sandford, S.A., Horz, F., Zolensky, M.E.

Wild2 and interstellar sample collection and Earth return. JGR108

E10, 8111, 2003.

Tuzzolino, A.J., Economou, T.E., Clark, B.C., Tsou, P., Brownlee,

D.E., Green, S.F., McDonnell, J.A.M., McBride, N., Colwell,

T.S.H. Dust measurements in the Coma of Comet 81P/Wild2 by

the dust flux monitor instrument. Science 304, 1776–1780, 2004.

Westphal, A.J., Snead, C., Butterwoorth, A., Graham, G.A., Bradley,

J.P., Bajt, S., Grant, P.G., Bench, G., Brennan, S., Pianetta, P.

Aerogel keystones: extraction of complete hypervelocity impact

events from aerogel collectors. Meteorit. Planet. Sci. 35, 1025–1041,

2000.

Wilke, M., Farges, F., Petit, P.-E., Brown Jr., G.E., Martin,

F. Oxidation state and coordination of Fe in minerals: an

Fe K-XANES spectroscopic study. Am. Min. 86, 714–730,

2001.